Profile measurement of concave spherical mirror and a flat mirror using a high-speed nanoprofiler

Abstract

Ultraprecise aspheric mirrors that offer nanofocusing and high coherence are indispensable for developing third-generation synchrotron radiation and X-ray free-electron laser sources. In industry, the extreme ultraviolet (wavelength: 13.5 nm) lithography used for high-accuracy aspheric mirrors is a promising technology for fabricating semiconductor devices. In addition, ultraprecise mirrors with a radius of curvature of less than 10 mm are needed in many digital video instruments. We developed a new type of nanoprofiler that traces the normal vector of a mirror's surface. The principle of our measuring method is that the normal vector at each point on the surface is determined by making the incident light beam on the mirror surface and the reflected beam at that point coincide, using two sets of two pairs of goniometers and one linear stage. From the acquired normal vectors and their coordinates, the three-dimensional shape is calculated by a reconstruction algorithm. The characteristics of the measuring method are as follows: the profiler uses the straightness of laser light without using a reference surface. Surfaces of any shape can be measured, and there is no limit on the aperture size. We calibrated this nanoprofiler by considering the system error resulting from the assembly error and encoder scale error, and evaluated the performance at the nanometer scale. We suppressed the effect of random errors by maintaining the temperature in a constant-temperature room within ±0.01°C. We measured a concave spherical mirror with a radius of curvature of 400 mm and a flat mirror and compared the results with those obtained using a Fizeau interferometer. The profiles of the mirrors were consistent within the range of system errors.

Accurate Inner Profile Measurement of a High Aspect Ratio Aspheric Workpiece Using a Two-Probe Measuring System

“…For example, in the rocket engine nozzles, the surface error and geometric error above the micrometer level will affect the stable flow state of the tail flame, resulting in the degradation of the engine performance [5]. A high aspect ratio aspheric workpiece is mainly produced by grinding or lapping processes [6,7]. Due to its high aspect ratio profile, it is normally a challenge to obtain high accuracy only with the manufacturing process without measurement since the control of multi-motion axes of the machine tool to track a high aspect ratio and large amplitude profile is always suffering from low accuracy compared with that of a smooth profile [8].…”

Cylindrical surface shape measurement method and measurement comparison

“…Therefore, in a previous study, we developed a nanoprofiler to guarantee high reproducibility and measurement accuracy 15 – 17 Thus far, we have been able to measure a spherical shape and an aspherical shape separately, but in this study, we newly measure a cylindrical surface shape as a free-form surface shape. Compared with other free-form surface shapes, the cylindrical surface shape has a less complex CGH pattern, and it is possible to achieve an interferometer measurement shape accuracy of 30 nm peak-to-valley (PV).…”

Three-dimensional surface figure measurement of high-accuracy spherical mirror with nanoprofiler using normal vector tracing method

“…Figure 1 illustrates the principle underlying the proposed profiler, which is based on the straightness of laser light and the high accuracy of rotational goniometers. 8,9 Our measurement method involves two components: an optical head and a sample motion unit. The optical head motion unit consists of two goniometers (rotating around the θ -and ϕ-axes), one linear motion stage (moving along the y-axis), and a sample motion unit consisting of two goniometers (rotating around the α-and β-axes).…”